Thanks for the very fast reply!
Good point. I just tested this. You're correct.
I could have thought about a possible interference between JMeter CLI and a listener writing files when I typed the "-l <output_file_name>" section of the command, but I didn't even consider that due to some statements from the user manual (see 1. below).

So the ticket as is can be closed, but you may want to read and consider the following three related points for future JMeter improvement:

1.
The user manual is a bit misleading here. It says (multiple times in slightly different ways):
If you want a results file to be written and not just shown some data in the GUI, you have to add a listener with a filename specified.
I didn't even consider that JMeter in CLI mode can write a results file without using the configured listeners for that as it does when run in GUI mode.

The user manual should probably state clearly somewhere:
If you run JMeter in GUI mode ALWAYS specify an output file for your results in at least one listener (if you specify "Filename" in multiple Listeners: a different file for each).
If you run JMeter in CLI mode, you have to specify a different filename in "-l <output_file_name>" than in any of your listeners.

2.
Due to the fact that JMeter (in CLI mode) is able to write an output file without (an explicitly specified) listener, it might be a good idea (for future changes), to have JMeter (in GUI mode) write a result file without the need for an output file configuration in a listener, too. Then you have CLI and GUI mode behave more equal to reduce confusion (and no one ever wants to run a test and end up without a results file).

3.
Is there any way to specify which fields should be recorded in the results file in CLI mode?
In GUI mode you can specify that in a listener ("Configure" in the "Filename" area), but I haven't seen that in the command line options or in a properties file.
If the result file fields (for CLI mode) are not configurable anywhere, that's an important missing feature IMHO, as a GUI-configured listener can currently log more fields than JMeter CLI logs as is.
(A workaround for this would be to have a listener write results (to a different file) too, and ignore the other results file.)